1. Field of the Invention
The present invention relates to rotary valves for controlling fluid flow. More particularly, this invention relates to a rotary valve with a valve member having a peripherally extending resilient overlay for sealing engagement with a peripheral seating surface on the valve body that is spaced from the valve stem axis.
2. Description of the Background
Rotary valves are distinct from other valves in that the valve member rotates within the valve body to create and break a seal with the seating surface, thereby effecting closing and opening of the valve. Rotary valves are frequently quarter turn valves, meaning that the valve member rotates 90.degree. between its fully closed to its fully opened positions.
One type of quarter turn rotary valve is referred to as a plug valve, which employs a valve element or plug having an external sealing surface and a cut-out or through port for transmitting fluid through the valve when the plug is rotated to its open position. Plug valves have long been preferred for controlling fluid flow in certain industries, particularly those wherein the fluid is a sludge or slurry, or when solid particles are transmitted through the valve with the fluid. U.S. Pat. No. 3,347,516 discloses such a plug valve with valve seat spaced from the stem axis. The valve element or plug has a frustoconical-shaped outer surface to seal with an inner surface of the valve body, with a cutout in the plug allowing fluid passage when the plug is rotated to its open position.
Another type of plug valve is marketed by Keystone Valve USA, Inc. under the "Ballcentric" trademark. The seating surface of this valve body is also spaced from the axis of the valve stem, and the plug has a generally hemispherical shape. When the eccentric plug is rotated to its closed position, the external wall of the plug seals with the annular seating surface on the valve body. When the plug is rotated 90.degree., the valve element is moved to the side of the valve body and out of the flow path to allow unobstructed flow through the valve. The Ballcentric valve improves flow characteristics through the valve and reduces pressure drop across the valve compared with plug valves having a conventional passageway through the plug. In order to improve valve sealing reliability, the metallic valve element has been coated with an elastomeric overlay so that the overlay seals with the seating surface to obtain a fluid-tight seal when the valve is in its closed position.
U.S. Pat. No. 2,852,226 discloses a plug valve with a flexible sealing plate provided on the plug body. The plug is of the traditional type with a passageway through the plug to allow fluid flow through the valve. The sealing plate includes a circumferential flange intended to be continuously pressed against the inner surface of the plug valve. The specification indicates that fluid pressure will urge the flange into tighter sealing engagement with the fluid line, and a pair of sealing plates are provided on opposing sides of the valve stem.
U.S. Pat. No. 3,404,864 discloses a rotary valve wherein the valve element is in the shape of a tube. As shown in FIGS. 13 and 14, a valving tube may be fitted over a valve extension tube, with an external periphery of the valving tube forming a lip that is biased by pressure toward the wall.
U.S. Pat. No. 3,990,676 discloses a sealing gasket on a valve element. The sealing gasket may include opposing lips for shutting off flow of fluid under pressure from either of the upstream port or downstream port of the valve.
Plug valves are preferably designed so that the valve element seals fluid flow when the valve is closed regardless of the direction fluid pressure is applied to the valve. Plug valves may be installed with the intent of fluid passing primarily in one direction through the valve, e.g., from an upstream high pressure side of the valve to a downstream low pressure side of the valve. The valve should ideally be able to seal fluid flow in the reverse direction, however, so that a leak occurring between two plug valves in a system can be isolated, with high pressure being maintained on opposing sides of the two closed plug valves.
One of the problems particularly associated with rotary valves having seating surfaces offset from the valve stem axis concerns the difficulty of maintaining sealing engagement between the plug and the seat when fluid pressure is pressing the plug toward the valve stem axis and thus away from the seat, while maintaining long seal life if opposing fluid pressure forces the plug toward tighter sealing engagement with the seat. This problem is compounded when the valve member includes a resilient overlay to enhance sealing reliability, as explained above, since the metallic plug body moves with respect to the valve stem axis in response to the fluid pressure differential, and since the resilient overlay also moves with respect to the metallic plug body in response to this same pressure differential across the closed valve. Traditional techniques for dealing with this dilemma have included the use of harder or less resilient overlays on the plug, increased plug-to-seat interference, and/or reduction in the "play" or maximum axial movement of the plug with respect to the seat. However, the benefits of an overlay are reduced if the elastomer becomes less resilient. Increased plug-to-seat inference results in increased torque requirements to operate the valve, thereby resulting in larger and more expensive stems, bearings, operators, and related components. Reducing play between the components results in increased manufacturing costs, use of more durable, and thus more expensive, components, and more frequent valve maintenance.
Thus, there has been a long-felt but unfulfilled need for an improved rotary valve that offers high sealing reliability and long seal life with a resilient overlay on a valve element adapted for engagement with a seat spaced from the valve stem axis.
The disadvantages of the prior art are overcome by the present invention, and an improved rotary valve is hereinafter disclosed for reliably sealing fluid flow. The techniques of the present invention are particularly well suited for use with a plug valve having a resilient overlay on a rigid valve body wherein the annular seating surface is offset from the valve stem and the plug responds to varying fluid pressure by slight axial movement.